46 WRITINGS OF JOSEPH HENRY. [1831 



from this it appears, that 6 short wires are more powerful 

 than 3 of double the length. 



Exp. 19. The wires used in Exp. 10, but united so as to 

 form a single coil of 120 feet of wire, lifted 60 lbs.; while in 

 Exp. 10, the weight lifted was 200 lbs. : this is a confirma- 

 tion of the result in the last experiment. 



Exp. 20. The same wires used in the last experiment 

 were attached to a small compound battery, consisting of 

 two plates of zinc and two of copper, after the plan of Prof. 

 Hare, and containing exactly the same quantity of zinc sur- 

 face, as the element in the last experiment; in this case the 

 weight lifted was 110 lbs., or nearly double that in the last. 

 This result is in strict accordance with that of Exp. 7 ; the 

 two plates having more "projectile force," and thus produc- 

 ing a greater effect with a long wire. 



In these experiments a fact was observed, which appears 

 somewhat surprizing: when the large battery was attached 

 and the armature touching both poles of the magnet, it was 

 capable of supporting more than 700 lbs. but when only 

 one pole is in contact it did not support more than 5 or 6 

 lbs., and in this case we never succeeded in making it lift 

 the armature (weighing 7 lbs.). This fact may perhaps be 

 common to all large magnets, but we have never seen the 

 circumstance noticed of so great a difference between a 

 single pole and both. 



A number of experiments were also made with reference 

 to the best form of the iron to receive magnetism, but no 

 very satisfactor}'- results were obtained; of these however, 

 the following are considered as not uninteresting. 



Exp. 21. A cylindrical bar of iron weighing 13 oz. A.^ 

 drachms, and bent into a horse-shoe, was covered with two 

 coils of wire each 60 feet long; with the small battery used 

 in the last experiment, it lifted 42 lbs. 



Exp. 22. A rectangular flat bar \^ of an inch wide, and ^ 

 of an inch thick, also bent into a horse-shoe, weighing 9 oz. 

 3 dr., and of exactly the same surface as the bar used in the 

 last experiment, with the same wires and battery, lifted 85 

 lbs 



